DECLARE @xml XML

SET @xml = 
    (
        SELECT
            DocumentId
        ,   KeywordValue = CAST('<![CDATA[' + ISNULL(KeywordValue,'') + ']]>' AS xml)
        FROM
            Webkeyword
        WHERE
            DocumentId = @DocumentId

        FOR XML AUTO, ELEMENTS, ROOT('DocumentKeywords')
  )

SELECT @xml
